1. Field of the Invention
This invention relates to a system for the formation of a silicon thin film and a good-quality semiconductor-insulating film interface. Such silicon thin films are used for crystalline silicon thin film transistors, and such semiconductor-insulating film interfaces are employed for field effect transistors. The invention also relates to a semiconductor thin film forming system by the pulsed laser exposure method. In addition, the invention relates to a system for the manufacture of driving elements or driving circuits composed of the semiconductor thin films or field effect thin film transistors for displays and sensors, for example.
2. Description of the Related Art
Typical processes for the formation of a thin film transistor (TFT) on a glass substrate are a hydrogenated amorphous silicon TFT process and a polycrystalline silicon TFT process. In the former process, the maximum temperature in a manufacture process is about 300° C., and the carrier mobility is about 1 cm2/Vsec. Such a hydrogenated amorphous silicon TFT formed by the former process is used as a switching transistor of each pixel in an active matrix (AM) liquid crystal display (LCD) and is driven by a driver integrated circuit (IC, an LSI formed on a single crystal silicon substrate) arranged on the periphery of a screen. Each of the pixels of this system includes an individual switching element TFT, and this system can yield a better image quality with less crosstalk than a passive matrix LCD. In such a passive matrix LCD, an electric signal for driving the liquid crystal is supplied from a peripheral driver circuit. In contrast, the latter polycrystalline silicon TFT process can yield a carrier mobility of 30 to 100 cm2/Vsec by, for example, employing a quartz substrate and performing a process at high temperatures of about 1000° C. as in the manufacture of LSIs. For example, when this process is applied to a liquid crystal display manufacture, such a high carrier mobility can yield a peripheral driver circuit on the same glass substrate concurrently with the formation of pixel TFTs for driving individual pixels. This process is therefore advantageous to minimize manufacture process costs and to downsize the resulting products. If the product should be miniaturized and should have a higher definition, a connection pitch between an AM-LCD substrate and a peripheral driver integrated circuit must be decreased. A conventional tab connection method or wire bonding method cannot significantly provide such a decreased connection pitch. However, if a process at high temperatures as in the above case is employed in the polycrystalline silicon TFT process, low softening point glasses cannot be employed. Such low softening point glasses can be employed in the hydrogenated amorphous silicon TFT process and are available at low costs. The process temperature in the polycrystalline silicon TFT process should be therefore decreased, and techniques for the formation of polycrystalline silicon films at low temperatures have been developed by utilizing a laser-induced crystallization technique.
Such a laser-induced crystallization is generally performed by a pulse laser irradiator having a configuration shown in FIG. 15. A laser light supplied from a pulse laser source 1101 reaches a silicon thin film 1107, a work, on a glass substrate 1108 via an optical path 1106. The optical path 1106 is specified by a group of optic devices including mirrors 1102, 1103, and 1105, and a beam homogenizer 1104. The beam homogenizer 1104 is arranged to uniformize spatial intensities of laser beams. Generally, because the irradiation area is smaller than that of the glass substrate 1108, the glass substrate on an X-Y stage 1109 is moved to irradiate an optional position on the substrate with a laser beam. The laser irradiation can be also performed by moving the optic device group or moving the optic device group and the stage in combination. Laser irradiation may also be carried out in a vacuum or in the high purity gas atmosphere within the vacuum chamber. When necessary, a cassette 1110 having the glass substrate with silicon thin film and a substrate carrier mechanism 1111 are provided for mechanically separating and accommodating the substrate between the cassette and the stage.
Japanese Patent Publication (JP-B) No. 7-118443 discloses a technique of irradiating an amorphous silicon thin film on an amorphous substrate with a short wavelength pulse laser light. This technique can crystallize an amorphous silicon while keeping the overall substrate from high temperatures, and can produce semiconductor elements or semiconductor integrated circuits on large substrates available at low costs. Such large substrates are required in liquid crystal displays, and such substrate available at low costs may be glasses, for example. However, as is described in the above publication, the crystallization of an amorphous silicon thin film by action of a short wavelength laser light requires an irradiation intensity of about 50 to 500 mJ/cm2. However, the maximum emission output of a conventionally available pulse laser irradiator is at most about 1 J/pulse, and an area to be irradiated by a single irradiation is at most about 2 to 20 cm2, by a simple conversion. For example, if the overall of a 47 cm×37 cm substrate should be crystallized by action of laser, at least 87 to 870 points of the substrate must be irradiated with a laser light. Likewise, the number of points to be irradiated with a laser light increases with an increasing size of the substrate, for example, as in a 1 m×1 m substrate. Such a laser-induced crystallization is generally performed by a pulse laser irradiator having a configuration shown in FIG. 15.
To form uniform thin film semiconductor elements on a large substrate by the above technique, an effective process is known as disclosed in Japanese Unexamined Patent Publication (JP-A) No. 5-211167 (Japanese Patent Application No. 3-315863). The process includes the steps of dividing the elements to portions smaller than the beam size of the laser and repeating a combination of irradiation with several pulses and movement of the area to be irradiated by step-and-repeat drawing method. In the process, the lasing and the movement of a stage (i.e., the movement of a substrate or laser beam) are alternatively performed, as shown in FIG. 16(2). However, even according to this process, the variation of lasing intensity exceeds ±5% to ±10% when the irradiation procedure is repeated at a density of about 1 pulse per irradiated portion to 20 pulses per irradiated portion using a currently available pulse laser irradiator with a uniformity of lasing intensity of ±5% to ±10% (in continuous lasing). The resulting polycrystalline silicon thin film and polycrystalline silicon thin film transistor cannot therefore have satisfactorily uniform characteristics. Particularly, the generation of a strong or weak light caused by an unstable discharge at early stages of lasing significantly invites such heterogeneous characteristics. This phenomena is called as spiking. As a possible solution to the spiking, a process of controlling an applied voltage in a subsequent lasing with reference to the results of integrated strengths can be employed. However, according to this process, a weak light is rather oscillated even though the formation of spiking is inhibited. Specifically, when irradiation periods and non-lasing periods are alternated, the intensity of a first irradiated pulse in each irradiation period is most unstable and is varied, as shown in FIG. 17. In addition, the history of irradiation intensity differs from point to point to be irradiated. The resulting transistor element and thin film integrated circuit cannot have a significant uniformity in the substrate plane.
To avoid such a spiking, a process is known to start lasing prior to the initiation of irradiation to an area for the formation of element, as shown in FIG. 16(2). However, this technique shown in FIG. 16(2) cannot be applied to a process of intermittently repeating the lasing and the movement of stage. To avoid these problems, a process is proposed in Japanese Unexamined Patent Publication (JP-A) No. 5-90191. The process includes the steps of allowing a pulse laser source to continuously oscillate and inhibiting irradiation of a substrate with the laser light by an optic shielding system during the movement of the stage. Specifically, as shown in FIG. 16(3), a laser is continuously oscillated at a predetermined frequency, and the movement of stage to a target irradiation position is brought into synchronism with the shielding of an optic path. By this configuration, a laser beam with a stable intensity can be applied to a target irradiation position. However, although this process can stably irradiate the substrate with a laser beam, the process also yields increased excess lasing that does not serve to the formation of a polycrystalline silicon thin film. The productivity is decreased from the viewpoint of the life of an expensive laser source and an excited gas, and the production efficiency of the polycrystalline silicon thin film is deteriorated with respect to power required for lasing. The production costs are therefore increased. When a substrate to be exposed to laser is irradiated with an excessively strong light as compared with a target intensity, the substrate will be damaged. Such an excessively strong light is induced by an irregular irradiation intensity. In LCDs and other imaging devices, a light passing through the substrate scatters in an area where the substrate is damaged, and the quality of image is deteriorated.
A process is known for the laser irradiation. In this process, a plurality of pulses are applied while the irradiation of each pulse is retarded. This process is disclosed by Ryoichi Ishihara et al. in “Effects of light pulse duration on excimer laser crystallization characteristics of silicon thin films”, Japanese Journal of Applied Physics, vol. 34, No. 4A, (1995), pp 1759 (hereinafter called as Reference). According to this Reference, the crystallization solidification rate of a molten silicon in a laser recrystallization process is 1 m/sec or more. To achieve a satisfactory growth of crystals, the solidification rate must be reduced. By applying a second laser pulse immediately after the completion of solidification, the second irradiation of laser pulse can yield a recrystallization process with a lower solidification rate. In viewing a temperature change (a time-hysteresis curve) of silicon as shown in FIG. 18, the temperature of silicon increases with the irradiation of laser energy, for example, as a pulse with an intensity shown in FIG. 19. When a starting material is an amorphous silicon (a-Si), the temperature further increases after the melting point of a-Si, and when the supplied energy becomes less than the energy required for increasing the temperature, the material begins to undergo cooling. At the solidifying point of a crystalline Si, the solidification proceeds for a solidification time and then completes, and the material is cooled to an atmospheric temperature. Provided that the solidification of silicon proceeds in a thickness direction from an interface between silicon and the substrate, an average solidification rate is calculated according to the following equation.Average solidification rate=(Thickness of silicon)/(Solidification time)
Specifically, if the thickness of silicon is constant, the solidification time is effectively prolonged to reduce the solidification rate. If the process maintains ideal conditions on thermal equilibrium, the solidification time can be prolonged by increasing an ideally supplied energy, i.e., a laser irradiation energy. However, as pointed out in the above reference, such an increased irradiation energy invites the resulting film to become amorphous or microcrystalline. In an actual melting and recrystallization process, the temperature does not change in an ideal manner as shown in FIG. 18, and the material undergoes overheating when heated and undergoes supercooling when cooled, and attains a stable condition. Particularly, when the cooling rate in cooling procedure is extremely large and the material undergoes an excessive supercooling, the material is not crystallized at around its solidification point, and becomes an amorphous solid due to quenching and rapid solidification. Under some conditions, thin films are converted not into amorphous materials but into microcrystals, as shown in the above-mentioned Reference.
Accordingly, an object of the invention is to provide a process for forming a semiconductor thin film with a lower trap state density by light irradiation and to provide a process and system for applying the above process to large substrates with a high reproducibility.
Another object of the invention is to provide a means for forming a satisfactory gate insulating film on the semiconductor thin film of good quality and to provide a system for producing a field effect transistor having a satisfactory semiconductor-insulating film interface, i.e., satisfactory properties.